202 A. Harher — Lamprophijres of North of England. 



that offered. Again, Mr. Houghton's analyses of two rocks from 

 the same locality on Docker Fell show a sharp contrast, their silica- 

 percentages differing by more than 10. Professor Bonney concludes 

 that the two specimens cannot be really from the same dyke, but 

 Mr. Collins in his analyses of Cornish lamprophyres shows an even 

 greater difference between two specimens taken in situ from one 

 mass. 



A few words on lamprophyres in general will not be out of place 

 at this point. From quite early days such rocks as miuette and 

 kersantite have been recognized as interesting types, not very 

 sharply divided from one another, but collectively occupying a 

 position somewhat apart from what may be regarded as more normal 

 igneous rocks. It is true that the principle of classifying rocks 

 by a mere enumeration of their constituent minerals has led some 

 geologists to confuse these types with the mica-bearing syenites and 

 diorites ; but such a view is not in harmony with either chemical 

 relationships or geological occurrence. To the field-geologist the 

 rocks in question have always been known as characteristically 

 "dyke-rocks"; more recently they have been shown to occur also 

 as special marginal fades of certain deep-seated bodies of rock. 



Eosenbusch (1887) distinctly recognizes the individuality of the 

 group, for which he adopts von Giimbel's name lamprophyre.^ He 

 points out its peculiarities, and separates from the two types already 

 mentioned two others, under the names vosgesite and camptonite, 

 in which the dark mica is more or less replaced by hornblende or 

 augite. He makes, however, a division of the group into a 'syenitic' 

 and a ' dioritic ' family, which seems to be quite artificial. It is 

 noteworthy that most of the best known lamprophyres are found ia 

 association not with syenites or diorites, but with granites. A 

 glance over Rosenbusch's lists of localities makes this fact at once 

 apparent. In what follows, the lamprophyres will be regarded not 

 as an independent group, but as a special basic modification of rocks 

 of the normal plutonic series. This point of view is scarcely a novel 

 one. Thus we find Hunter and Kosenbuisch ^ describing as a new 

 type " monchiquite, a camptonitic dyke-rock associated with the 

 eh\5olite-syenites " of Brazil and Portugal, while J. F. Williams^ 

 has given an account of such rocks and others (fourchite and 

 ouacliitite) in Arkansas, and has demonstrated their genetic relations 

 with the elfeolite-syenites of that state. The varied series of rocks 

 studied by Brogger in the Christiania district seem in several 

 instances to run to lamprophyric modifications, and we may expect 

 much light to be thrown on the subject in that eminent geologist's 

 forthcoming monograph. 



In endeavouring to explain the multiplicity of igneous rocks, 

 and the evident genetic relations between widel}' different types, 

 geologists have been led to speculate on the separation, by gravity 

 or otherwise, of a large reservoir of molten magma into more acid 



' The name mica-trap evidently cannot be made to cover all the types here included. 

 ' Tsch. Min. Mitth. (2) vol. xi. p. 445. 

 3 Eep. Geol. Surv. Ark. for 1890, vol. i. 



